The present invention relates to equipment used in subterranean operations and, in particular, to systems and methods of supporting a multilateral window or casing joint from collapse.
Hydrocarbons can be produced through relatively complex wellbores traversing a subterranean formation. Some wellbores can include multilateral wellbores and/or sidetrack wellbores. Multilateral wellbores include one or more lateral wellbores extending from a parent (or main) wellbore. A sidetrack wellbore is a wellbore that is diverted from a first general direction to a second general direction and can include a main wellbore in the first general direction and a secondary wellbore diverted from the main wellbore in the second general direction. A multilateral wellbore can include one or more windows or casing exits to allow corresponding lateral wellbores to be formed. A sidetrack wellbore can also include a window or casing exit to allow the wellbore to be diverted to the second general direction.
The casing exit for either multilateral or sidetrack wellbores can be formed downhole by positioning a casing joint and a whipstock in a casing string at a desired location in the main wellbore. The whipstock is used to deflect one or more mills laterally (or in an alternative orientation) relative to the casing string. The deflected mill(s) penetrates part of the casing joint to form the casing exit and drill bits can be subsequently inserted through the casing exit in order to drill the lateral or secondary wellbore.
Casing joints are often made from high-strength, non-corrosive materials that are able to withstand corrosive elements present in the subterranean environment, such as hydrogen sulfide and carbon dioxide. Milling the high-strength material to form the casing exit, however, can be difficult and usually creates a large amount of cutting debris that can detrimentally affect well completion and hydrocarbon production operations. For example, accumulated cutting debris can obstruct the retrieval of the whipstock, plug flow control devices, damage seals, obstruct seal bores, and interfere with positioning components in the main bore below the casing joint.
To avoid the accumulation of excessive cutting debris downhole, the casing exit or window is sometimes pre-milled into the casing joint before it is introduced into the wellbore. In such applications, an outer liner is often used to cover the pre-milled window and thereby prevent the influx of particulate materials into the interior of the casing string. The outer liner is typically made of fiberglass or other soft materials and therefore can be milled through quite easily once appropriately positioned downhole.
In the downhole environment, and especially during cementing completion operations, the casing joint often experiences high pressures. To prevent the outer liner from collapsing into the pre-milled window as a result of these pressures, a sleeve is often provided about the exterior of the casing joint at the location of the window. The thought is that the sleeve should able to increase the collapse pressure rating of the window beyond what the outer liner is able to provide. In cases where there is no pre-milled window, the sleeve may also be used on the interior of the casing joint in order to increase the collapse pressure rating of the casing joint itself.
Nonetheless, there are times where the downhole pressures are so extreme that the sleeve is not sufficient to prevent collapse of the outer liner or casing joint. One method to resolve this problem is to increase the wall thickness of the sleeve or place multiple concentric sleeves at the window or within the casing joint. However, these methods can be prohibitive since added radial size either inwardly or outwardly to the casing joint may pose unreasonably large structural obstructions that can complicate and/or prevent subsequent downhole operations from being accomplished.